Lidar isnâ€™t a word most people use regularly, but recent developments in the field might see a future where is becomes part of everyday life.
Lidar, an acronym for LIght Detection And Ranging, was first developing in the 1960â€™s and is primarily a technique for measuring distance; however, other applications include atmospheric Lidar which measures clouds, particles and gases such as ozone. The system comprises of a laser, a scanner and GPS position receiving, and it works by emitting a laser pulse towards a target, and measuring the time it takes for the pulse to return.
There are two main types of Lidar used within remote sensing for measuring distance, topographic and bathymetric; topographic Lidar uses a near infrared laser to map land, while bathymetric Lidar uses water-penetrating green light to measure the seafloor. The image at the top of the blog is a bathymetric Lidar overlaying an aerial photograph Pinellas Point, Tampa Bay in the USA, showing depths below sea level in metres. Airborne terrestrial Lidar applications have also been expanded to include measuring forest structures and tree canopies mapping; whilst thereâ€™s ground based terrestrial laser scanners for mapping structures such as buildings.
As a user getting freely accessible airborne Lidar data isnâ€™t easy, but there are some places that offer datasets including:
- Channel Coast Observatory offers UK coastal Lidar going back to 2004.
- USGS EarthExplorer & NOAAâ€™s Digital Coast both offer Lidar data for the United States.
Spaceborne terrestrial Lidar has been limited, as it has to overcome a number of challenges:
- Itâ€™s an active remote sensing technique, which means it requires a lot more power to run, than passive systems and for satellites this means more cost.
- Itâ€™s an optical system that like all optical systems is affected by cloud cover and poor visibility, although interestingly it works more effectively at night, as the processing doesnâ€™t need to account for the sunâ€™s reflection.
- Lidar performance decreases with inverse square of the distance between the target and the system.
- Lidar collects individual points, rather than an image, and images are created by combining lots of individual points. Whilst multiple overflies are possible quickly in a plane, with a satellite orbiting the Earth youâ€™re effectively collecting lines of points over a number of days, which takes time.
The only satellite that studied the Earthâ€™s surface using Lidar is NASAâ€™s Ice, Cloud and Land Elevation Satellite – Geoscience Laser Altimeter system (IceSAT-GLAS); launched in 2003, it was decommissioned in 2010. It measured ice sheet elevations and changes, together with cloud and aerosol height profiles, land elevation and vegetation cover, and sea ice thickness; and you find its data products here. IceSAT-GLAS 2 is scheduled for launch in 2017. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), part of the satellite A-Train, is a joint NASA and CNES mission launched in 2006. Originally designed as an atmospheric focused Lidar, it has since developed marine applications that led to the SABOR campaign we discussed in previous blog.
Beyond remote sensing, Lidar may become part of every household in the future, if recent proof-of-concepts come to fruition. The Google self-drive car uses a Lidar as part of its navigation system to generate a 3D maps of the surrounding environment. In addition, research recently published in Optics Express, by Dr. Ali Hajimiri of California Institute of Technology has described the potential of a tiny Lidar device capable of turning mobile phones into 3D scanning devices. Using a nanophotonic coherent imager, the proof-of-concept device has put together a 3-D image of the front of a U.S. penny from half a meter away, with 15-Î¼m depth resolution and 50-Î¼m lateral resolution.
Lidar has many remote sensing and surveying applications, however, in the future we all could have lasers in our garage and pockets.